A disc drive apparatus records an information signal onto and reproduces it from a disc-shaped recording medium such as an optical disc. Such a disc drive apparatus comprises an optical pickup for irradiating a disc-shaped recording medium with a laser beam by movement of the optical pickup in radial directions of the disc-shaped recording medium.
An objective lens drive device is disposed in the optical pickup. The objective lens drive device moves an objective lens, which is held by a movable block, to perform focusing adjustment in focusing directions, and moves the objective lens in tracking directions to perform tracking adjustment, so that a spot of a laser beam with which the disc-shaped recording medium is irradiated is focused on a recording track of the disc-shaped recording medium through the objective lens. The focusing directions correspond to directions in which the objective lens moves into contact with and separates from a recording surface of the disc-shaped recording medium. The tracking directions correspond to substantially radial directions of the disc-shaped recording medium.
In such an optical pickup, focusing adjustment and tracking adjustment are generally performed with an objective lens drive device. In recent years, in order to increase the followability of a laser beam spot with respect to a recording track, an objective lens drive device that is called a triaxial actuator has been developed. This type of objective lens drive device performs, in addition to bi-axial focusing and tracking adjustments, an adjustment, for example, when a surface of a rotating disc-shaped recording medium is deflected, with a movable block being tiltable with respect to a recording surface of the disc-shaped recording medium.
The following types of objective lens drive device that are called triaxial actuators have been developed.
In a moving-coil objective lens drive device, a movable block, which holds an objective lens, is connected to a stationary block through a supporting spring, and a tilt coil for tilting the movable block with respect to the stationary block is disposed at the movable block. (For example, Japanese Unexamined Patent Application Publication No. 2000-149292 (Patent Document) should be referred to.)
In the moving-coil objective lens drive device having a tilt coil disposed at the movable block, a focusing coil for performing focusing adjustment and a tracking coil for performing tracking adjustment are disposed separately from the tilt coil at the movable block. Therefore, two supporting springs are required for each coil for supplying drive current to the coils. Consequently, the movable block is supported at the stationary block with a total of six supporting springs.
In the moving-coil objective lens drive device, magnets oppose the focusing coil, the tracking coil, and the tilt coil, which are disposed at the movable block. Accordingly, drive current is supplied to the focusing coil, the tracking coil, and the tilt coil through the supporting springs. At this time, a thrust is generated in a predetermined direction by the coils and the magnets. Therefore, the movable block is moved with respect to the stationary block in a required direction to carry out adjustments.
In a moving-magnet objective lens drive device, which is a type of tri-axial actuator that is different from the moving-coil type, a movable block, which holds an objective lens, is connected to a stationary block through a supporting spring, and a tilt magnet for tilting the movable block with respect to the stationary block is disposed at the movable block.
In the moving-magnet objective lens drive device having a tilt magnet disposed at the movable block, a focusing coil, a tracking coil, and a tilt coil are disposed at the stationary block so that they oppose magnets at the movable block.
In the moving-magnet objective lens drive device, drive current is supplied to the focusing coil, the tracking coil, or the tilt coil, all of which are disposed at the stationary block, through a single-purpose electric supply line. At this time, a thrust is generated in a predetermined direction by the coils and the magnets. Therefore, the movable block is moved with respect to the stationary block in a required direction to carry out adjustments.
However, the above-described related objective lens drive devices that are adjustable in three axial directions have the following problems.
In the moving-coil objective lens drive device, two supporting springs for supplying electrical current to the tilt coil are required in addition to the four supporting springs required for focusing adjustment and tracking adjustment. Therefore, the stationary block and the movable block that have been connected together with four supporting springs considerably lose balance. Consequently, it is necessary to design the objective lens drive device so that the state of connection of the stationary and movable blocks is equivalent to that using four supporting springs. In addition, since it is not easy to design the objective lens drive device in such a way, production costs of such objective lens drive devices are increased.
Further, since the movable block is additionally provided with a tilt coil, the movable block is heavier and, thus, has reduced sensitivity.
Still further, since three types of coils are disposed at the movable block, for example, the specification of magnets opposing these coils is changed, thereby making it difficult to reduce the size and thickness of the objective lens drive device.
In order to increase the sensitivity of the movable block and drive it in a tilting manner, the rigidity of the supporting springs needs to be low. Therefore, the drive resonance frequency is decreased, and, depending upon the type of disc-shaped recording medium, an error in recording an information signal may occur.
On the other hand, in the moving-magnet objective lens drive device, since magnets having a relatively high specific gravity are disposed at the movable block, the moving block is particularly heavy, thereby considerably reducing its sensitivity. In order to overcome the problem of a reduction in sensitivity, it is necessary to use magnets that produce large magnetic force and to supply a large drive current, thereby resulting in problems such as an increase in production costs and power consumption of moving-magnet objective lens drive devices.
In addition, since three types of coils are disposed at the stationary block, the moving-magnet objective lens drive device is less easily assembled and is increased in size.
Accordingly, it is an object of the present invention to provide an optical pickup and a disc drive apparatus which make it possible to overcome the aforementioned problems, so that, with an objective lens drive device having good characteristics, the followability of a laser beam spot with respect to a recording track is improved.